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Topic Review
Green Star
In astronomy, a green star is a white or blueish star that appears greenish in some viewing conditions (see § Psychology below). Under typical viewing conditions, there are no greenish stars, because the color of a star is more or less given by a black-body spectrum. However, there are a few stars that appear greenish to some observers, due to the viewing conditions, for example the optical 'illusion' that a red object can make nearby objects look greenish (and vice versa). Some multiple star systems, such as Antares, have a bright reddish star where this contrast makes other stars in the system seem greenish.
  • 2.2K
  • 21 Nov 2022
Topic Review
Compact Fusion Reactor
The Lockheed Martin Compact Fusion Reactor (CFR) is a proposed nuclear fusion reactor project at Lockheed Martin’s Skunk Works. Its high-beta configuration, which implies that the ratio of plasma pressure to magnetic pressure is greater than or equal to 1 (compared to tokamak designs' 0.05), allows a compact fusion reactor (CFR) design and expedited development. The CFR chief designer and technical team lead, Thomas McGuire studied fusion as a source of space propulsion in response to a NASA desire to improve travel times to Mars.
  • 477
  • 21 Nov 2022
Biography
Friedrich Hund
Friedrich Hermann Hund (4 February 1896 – 31 March 1997) was a Germany physicist from Karlsruhe known for his work on atoms and molecules.[1] Hund worked at the Universities of Rostock, Leipzig, Jena, Frankfurt am Main, and Göttingen. Hund worked with such prestigious physicists as Schrödinger, Dirac, Heisenberg, Max Born, and Walter Bothe. At that time, he was Born's assistant, working
  • 943
  • 18 Nov 2022
Topic Review
Virgo Interferometer
The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. Virgo is a Michelson interferometer that is isolated from external disturbances: its mirrors and instrumentation are suspended and its laser beam operates in a vacuum. The instrument's two arms are three kilometres long and located near Pisa, Italy. Virgo is part of a scientific collaboration of laboratories from six countries: Italy and France (the two countries behind the project), the Netherlands, Poland, Hungary and Spain. Other interferometers similar to Virgo have the same goal of detecting gravitational waves, including the two LIGO interferometers in the United States (at the Hanford Site and in Livingston, Louisiana). Since 2007, Virgo and LIGO have agreed to share and jointly analyze the data recorded by their detectors and to jointly publish their results. Because the interferometric detectors are not directional (they survey the whole sky) and they are looking for signals which are weak, infrequent, one-time events, simultaneous detection of a gravitational wave in multiple instruments is necessary to confirm the signal validity and to deduce the angular direction of its source. The interferometer is named for the Virgo Cluster of about 1,500 galaxies in the Virgo constellation, about 50 million light-years from Earth. As no terrestrial source of gravitational wave is powerful enough to produce a detectable signal, Virgo must observe the Universe. The more sensitive the detector, the further it can see gravitational waves, which then increases the number of potential sources. This is relevant as the violent phenomena Virgo is potentially sensitive to (coalescence of a compact binary system, neutron stars or black holes; supernova explosion; etc.) are rare: the more galaxies Virgo is surveying, the larger the probability of a detection.
  • 460
  • 18 Nov 2022
Topic Review
Quantum-Optical Spectroscopy
Quantum-optical spectroscopy is a quantum-optical generalization of laser spectroscopy where matter is excited and probed with a sequence of laser pulses. Classically, such pulses are defined by their spectral and temporal shape as well as phase and amplitude of the electromagnetic field. Besides these properties of light, the phase-amplitude aspects have intrinsic quantum fluctuations that are of central interest in quantum optics. In ordinary laser spectroscopy, one utilizes only the classical aspects of laser pulses propagating through matter such as atoms or semiconductors. In quantum-optical spectroscopy, one additionally utilizes the quantum-optical fluctuations of light to enhance the spectroscopic capabilities by directly shaping and/or detecting the quantum fluctuations of light. Quantum-optical spectroscopy has applications in controlling and characterizing quantum dynamics of many-body states because one can directly access a large set of many-body states, which is not possible in classical spectroscopy.
  • 369
  • 17 Nov 2022
Topic Review
Onium Compound
In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH+4, the protonated derivative of ammonia, NH3. The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, (C6H5)4P+. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium. A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on. Compounds of an onium cation and some other negative ion are known as onium compounds or onium salts. Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:
  • 517
  • 16 Nov 2022
Topic Review
Frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in units of hertz (Hz) which is equal to one occurrence of a repeating event per second. The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute (2 hertz), its period, T, — the time interval between beats—is half a second (60 seconds divided by 120 beats). Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.
  • 3.9K
  • 16 Nov 2022
Topic Review
Neutral Beam Injection
Neutral beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field, and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 90s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative ion based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NBI designed for ITER is a substantial challenge (D0, 1MeV, 40A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), and ion cyclotron resonance heating (ICRH), Lower hybrid resonance heating (LH).
  • 802
  • 15 Nov 2022
Topic Review
AFM-IR
AFM-IR (atomic force microscope infrared-spectroscopy) is one of a family of techniques that are derived from a combination of two parent instrumental techniques; infrared spectroscopy and scanning probe microscopy (SPM). The term was first used to denote a method that combined a tuneable free electron laser with an atomic force microscope (a type of SPM) equipped with a sharp probe that measured the local absorption of infrared light by a sample; it required that the sample be coupled to an infrared-transparent prism and be less than 1μm thick. It improved the spatial resolution of photothermal AFM-based techniques from microns to circa 100 nm. Recording the amount of infrared absorption as a function of wavelength or wavenumber creates an infrared absorption spectra that can be used to chemically characterize and even identify unknown materials. Recording the infrared absorption as a function of position can be used to create chemical composition maps that show the spatial distribution of different chemical components. Novel extensions of the original AFM-IR technique and earlier techniques have enabled the development of bench-top devices capable of nanometer spatial resolution, that do not require a prism and can work with thicker samples, and thereby greatly improving ease of use and expanding the range of samples that can be analysed. One of these techniques has achieved spatial resolutions down to around 20 nm, with a sensitivity down to the scale of molecular monolayer AFM-IR is related to techniques such as tip-enhanced Raman spectroscopy (TERS), scanning near-field optical microscopy (SNOM), nano-FTIR and other methods of vibrational analysis with scanning probe microscopy.
  • 1.0K
  • 11 Nov 2022
Topic Review
Spectroelectrochemistry
Spectroelectrochemistry (SEC) is a set of multi-response analytical techniques in which complementary chemical information (electrochemical and spectroscopic) is obtained in a single experiment. Spectroelectrochemistry provides a whole vision of the phenomena that take place in the electrode process. The first spectroelectrochemical experiment was carried out by Theodore Kuwana, PhD, in 1964. The main objective of spectroelectrochemical experiments is to obtain simultaneous, time-resolved and in-situ electrochemical and spectroscopic information on reactions taking place on the electrode surface. The base of the technique consist in studying the interaction of a beam of electromagnetic radiation with the compounds involved in these reactions. The changes of the optical and electrical signal allow us to understand the evolution of the electrode process. The techniques on which the spectroelectrochemistry is based are: Spectroelectrochemistry provides molecular, thermodynamic and kinetic information of reagents, products and/or intermediates involved in the electron transfer process.
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